Buried pipes and electrical cable shields are subject to environmental degradation due to corrosion, ground settling, thermal expansion, or excessive fluid pressure. The physical and hermetic integrity of buried pipes and cable shields is essential to properly carry out their intended function. A pipe which transmits fluids under pressure will constitute a substantial danger if the thickness of the pipe casing is reduced in localized regions through the agency of corrosion. The cylindrical shield for buried electrical cables will contribute to electrical power failures if the shield casing is cracked so as to admit ground water into it thereby shorting out the cables. The types of flaws which can contribute to the failure of a pipe or an electrical cable shield can be small by conventional standards, producing variations in the linear electrical resistance in the casing of only a milliohm or less. Conventional techniques for detecting flaws in buried conductive pipe casings and cable shields will not reveal the presence of such a flaw.
One prior art approach to detecting flaws in pipe casings is to energize electric wires within the pipe casing and to move a detector along the outside of the buried pipe, above the ground. The conductive property of the pipe casing strongly attenuates the electromagnetic radiation from the wires within the pipe so that a very weak signal is transmitted to the outside of the pipe. By virtue of the inverse square attenuation of the radiation from the outside of the pipe to the ground level location of the detector, the originally weak transmitted signal becomes virtually undetectable and variations in that signal indicative of flaws in the milliohm or less range becomes completely undetectable.